1. Field of the Invention
The present invention is directed to a method and apparatus for calibrating image data from a given medical imaging protocol.
2. Description of the Prior Art
In the medical imaging field, several imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an injected radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions.
The Standardized Uptake Value (SUV) is a widely-used measure for quantifying radiotracer (especially 18F-FDG) uptake in clinical PET scans. This value is computed from the number of counts of emission events recorded per voxel in the image reconstructed from the event data captured in the PET scan (coincidence emission events along the line of response). Its use is intended to provide normalization for differences in patient size and body composition, along with the dose of radiotracer injected, thereby enabling inter-study comparison, both between and within individual patients.
While differences in body composition and injected dose represent one source of variation, differences in scanner hardware and reconstruction represent another, and these are not addressed by the use of SUV. These unaddressed sources of variation impede the acceptance of PET as a quantitative imaging tool for lesion characterization, prognostic stratification and treatment monitoring, since differences in scanner hardware and reconstruction can significantly impact generated SUV. For example, SUVs typically increase with the number of iterations performed for iterative reconstruction techniques such as OSEM. Also, post-reconstruction smoothing will reduce SUVs in areas of high uptake. As such, better standardization and improved comparability between scanners and reconstruction protocols are required.
A number of review articles addressing the issue of standardization have been published in recent years, and in general, fall into one of three categories:    1. The EORTC (Young et al., 1999, Measurement of clinical and subclinical tumor response using [18F]-FDG and PET: Review and 1999 EORTC recommendations, Eur J Can. 35 (13) 1773-1782), NCI (Shanker et al., 2006, Consensus recommendation for the use of 18F-FDG as an indicator of therapeutic response in patients in National Cancer Institute trials, JNM. 47 (6) 1059-1066) and SNM (Delbeke et al., 2006, Procedure guideline for tumor imaging with 18F-FDG PET/CT 1.0, JNM. 47 (5) 885-895) provide no specific recommendations for normalizing the effect of reconstruction or scanner hardware on SUV. Instead, they focus on standardizing the imaging procedure (i.e., interval between injection and acquisition, cross-calibration of dose counters, etc.).    2. The so called “Netherlands Protocol” (Boellaard et al., 2008, The Netherlands protocol for standardization and quantification of FDG whole body PET studies in multi-centre trials, Eur J Nuc Med Mol Imaging. 35 (12) 2320-2333) provides a very prescriptive protocol with a specific set of reconstruction parameters for one scanner from each of the main manufacturers, along with upper and lower bounds for the recovery coefficients expected with a modified NEMA Image Quality phantom. An updated version of these guidelines was recently published by Boellaard et al., (2010, FDG PET and PET/CT: EANM procedure guidelines for tumor PET imaging: version 1.0, Eur J Nuc Med Mol Imaging. 37, 181-200).    3. Weber et al. (2007, Monitoring cancer treatment with PET/CT: Does it make a difference?, JNM. 48 (1) 36S-44S) suggests providing only bounds for SUV measures on a given (i.e., NEMA-like) phantom rather than specifying reconstruction parameters.
The third alternative may be the most appealing from a manufacturer's perspective, since it offers the greatest flexibility, allowing the manufacturer to take the decision on the most suitable reconstruction configuration. However, this proposal would still require all manufacturers to reconstruct and display their images to conform to the lowest common denominator, removing any competitive advantages.
There is currently no apparent solution built by a hardware or a software manufacturer beyond these recommendations from the clinical literature.
While not addressing the issue of reconstruction-dependent variation in SUV, a variety of corrections for SUV are clinically used to correct for body composition and blood glucose concentration. These corrections incorporate patient measurements (e.g., height, weight, blood glucose concentration) and adjust the standard body-weight normalized SUV as a function of these patient-specific parameters.